Curvature-based face detector

ABSTRACT

A method for processing data includes receiving a depth map of a scene containing at least a humanoid head, the depth map comprising a matrix of pixels having respective pixel depth values. A digital processor extracts from the depth map a curvature map of the scene. The curvature map includes respective curvature values of at least some of the pixels in the matrix. The curvature values are processed in order to identify a face in the scene.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application 62/396,839, filed Sep. 20, 2016, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods and systems for three-dimensional (3D) mapping, and specifically to processing of 3D map data.

BACKGROUND

A number of different methods and systems are known in the art for creating depth maps. In the present patent application and in the claims, the term “depth map” refers to a representation of a scene as a two-dimensional matrix of pixels, in which each pixel corresponds to a respective location in the scene and has a respective pixel depth value, indicative of the distance from a certain reference location to the respective scene location. In other words, the depth map has the form of an image in which the pixel values indicate topographical information, rather than brightness and/or color of the objects in the scene. Depth maps may be created, for example, by detection and processing of an image of an object onto which a pattern is projected, as described in U.S. Pat. No. 8,456,517, whose disclosure is incorporated herein by reference. The terms “depth map” and “3D map” are used herein interchangeably and have the same meaning.

Depth maps may be processed in order to segment and identify objects in the scene. Identification of humanoid forms (meaning 3D shapes whose structure resembles that of a human being) in a depth map, and changes in these forms from scene to scene, may be used as a means for controlling computer applications. For example, U.S. Pat. No. 8,249,334, whose disclosure is incorporated herein by reference, describes a computer-implemented method in which a depth map is segmented so as to find a contour of a humanoid body. The contour is processed in order to identify a torso and one or more limbs of the body. An input is generated to control an application program running on a computer by analyzing a disposition of at least one of the identified limbs in the depth map.

As another example, U.S. Pat. No. 8,565,479, whose disclosure is incorporated herein by reference, describes a method for processing a temporal sequence of depth maps of a scene containing a humanoid form. A digital processor processes at least one of the depth maps so as to find a location of the head of the humanoid form, and estimates dimensions of the humanoid form based on this location. The processor tracks movements of the humanoid form over the sequence using the estimated dimensions.

U.S. Pat. No. 9,047,507, whose disclosure is incorporated herein by reference, describes a method that includes receiving a depth map of a scene containing at least an upper body of a humanoid form. The depth map is processed so as to identify a head and at least one arm of the humanoid form in the depth map. Based on the identified head and at least one arm, and without reference to a lower body of the humanoid form, an upper-body pose, including at least three-dimensional (3D) coordinates of shoulder joints of the humanoid form, is extracted from the depth map.

SUMMARY

Embodiments of the present invention provide methods, devices and software for extracting information from depth maps.

There is therefore provided, in accordance with an embodiment of the invention, a method for processing data, which includes receiving a depth map of a scene containing at least a humanoid head, the depth map comprising a matrix of pixels having respective pixel depth values. Using a digital processor, a curvature map of the scene is extracted from the depth map. The curvature map includes respective curvature values of at least some of the pixels in the matrix. The curvature values are processed in order to identify a face in the scene.

In some embodiments, processing the curvature values includes detecting one or more blobs in the curvature map over which the pixels have respective curvature values that are indicative of a convex surface, and identifying one of the blobs as the face. Typically, the curvature map includes respective curvature orientations of the at least some of the pixels, and identifying the one of the blobs includes calculating a roll angle of the face responsively to the curvature orientations of the pixels in the one of the blobs. In a disclosed embodiment, processing the curvature values includes applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated roll angle.

Additionally or alternatively, processing the curvature values includes calculating a scale of the face responsively to a size of the one of the blobs, and applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated scale.

Further additionally or alternatively, extracting the curvature map includes deriving a first curvature map from the depth map at a first resolution, and detecting the one or more blobs includes finding the one or more blobs in the first curvature map, and processing the curvature values includes deriving a second curvature map containing the one of the blobs at a second resolution, finer than the first resolution, and identifying the face using the second curvature map.

In some embodiments, processing the curvature values includes convolving the curvature map with a curvature filter kernel in order to find a location of the face in the scene. In a disclosed embodiment, convolving the curvature map includes separately applying a face filter kernel and a nose filter kernel in order to compute respective candidate locations of the face, and finding the location based on the candidate locations. Additionally or alternatively, convolving the curvature map includes computing a log likelihood value for each of a plurality of points in the scene, and choosing the location responsively to the log likelihood value.

There is also provided, in accordance with an embodiment of the invention, apparatus for processing data, including an imaging assembly, which is configured to capture a depth map of a scene containing at least a humanoid head, the depth map including a matrix of pixels having respective pixel depth values. A processor is configured to extract from the depth map a curvature map of the scene, the curvature map including respective curvature values of at least some of the pixels in the matrix, and to process the curvature values in order to identify a face in the scene.

There is additionally provided, in accordance with an embodiment of the invention, a computer software product, including a non-transitory computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a depth map of a scene containing at least a humanoid head, the depth map including a matrix of pixels having respective pixel depth values, to extract from the depth map a curvature map of the scene, the curvature map including respective curvature values of at least some of the pixels in the matrix, and to process the curvature values in order to identify a face in the scene.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a system for 3D mapping of humanoid forms, in accordance with an embodiment of the present invention;

FIG. 2 is a schematic representation of a depth map, layered with a predicted face blob, in accordance with an embodiment of the present invention;

FIG. 3 is a schematic representation of a normal map extracted from the depth map of FIG. 2 at low resolution, in accordance with an embodiment of the present invention;

FIG. 4 is a schematic representation of a coarse-level curvature map extracted from the normal map of FIG. 3, in accordance with an embodiment of the present invention;

FIG. 5 is a schematic representation of a map of blobs extracted from the curvature map of FIG. 4, in accordance with an embodiment of the invention;

FIG. 6 is a schematic representation of a map of curvature direction within the blobs found in FIG. 5, in accordance with an embodiment of the invention;

FIG. 7 is a schematic representation of a normal map extracted from the depth map of FIG. 2 at high resolution, in accordance with an embodiment of the present invention;

FIG. 8 is a schematic representation of a fine-grained curvature map extracted from the normal map of FIG. 7, in accordance with an embodiment of the present invention;

FIGS. 9A and 9B are schematic graphical representations of filter kernels used in face detection, in accordance with an embodiment of the invention; and

FIGS. 10A and 10B are schematic graphical representations of log likelihood maps obtained by convolving the curvature map of FIG. 8 with the filter kernels of FIGS. 9A and 9B, respectively, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

U.S. patent application Ser. No. 15/272,455, filed Sep. 22, 2016, whose disclosure is incorporated herein by reference, describes methods, systems and software for extracting humanoid forms from depth maps. In the disclosed methods, a digital processor extracts a curvature map from the depth map of a scene containing a humanoid form. The curvature map comprises respective oriented curvatures of at least some of the pixels in the depth map. In other words, at each of these pixels, the curvature map holds a scalar signed value indicating the dominant curvature value and the corresponding curvature orientation, i.e., the direction of the dominant curvature, expressed as a two-dimensional (2D) vector. The processor segments the depth map using both curvature values and orientations in the curvature map, and thus extracts 3D location and orientation coordinates of one or more limbs of the humanoid form.

The processor segments the depth map by identifying blobs in the curvature map over which the pixels have a positive curvature, meaning that the surfaces of these blobs are convex (although this definition of “positive” curvature is arbitrary, and curvature could alternatively be defined so that convex surfaces have negative curvature). The edges of the blobs are identified in the depth map at locations of sign changes in the curvature map. This use of curvature enhances the reliability and robustness of segmentation, since it enables the processor to distinguish between different blobs and between blobs and the background even when there is no marked change in depth at this edges of a given blob, as may occur when one body part occludes another, or when a body part is resting against a background surface or other object.

Embodiments of the present invention that are described herein process curvature maps specifically in order to identify one or more faces in the scene. Typically, in the disclosed methods, one or more blobs are detected in a curvature map as described above. The curvature orientations of the pixels in a blob that is a candidate to correspond to a face are processed in order to estimate the roll angle of the face. A curvature filter can then be applied to the curvature map while correcting for the calculated roll angle, in order to ascertain the likelihood that this blob is indeed a face. Additionally or alternatively, the size of the blob can be used to estimate and correct for the scale of the face.

Various sorts of classifiers can be used to extract faces from the curvature map. In some embodiments, which are described in greater detail hereinbelow, the curvature map is convolved with one or more curvature filter kernels in order to find the location of a face in the scene. In one embodiment, a face filter kernel and a nose filter kernel are applied separately in order to compute respective candidate locations, which are used in finding the actual face location. These filters are matched to the curvature features of a typical face (including the relatively high convex curvature of the nose), and are relatively insensitive to pitch and yaw of the face. The roll angle and scale can be normalized separately, as explained above. The filter can be configured to return a log likelihood value for each candidate point in the scene, whereby points having the highest log likelihood value can be identified as face locations.

FIG. 1 is a schematic, pictorial illustration of a system 20 for depth mapping and imaging, in accordance with an embodiment of the present invention. In this example, an imaging assembly 24 is configured to capture and process depth maps and images of a scene, which in this case contains a humanoid subject 36. An imaging assembly of this sort is described, for example, in the above-mentioned U.S. Pat. No. 8,456,517. The principles of the present invention are by no means limited to the sort of pattern-based mapping that is described in this patent, however, and may be applied in processing depth maps generated by substantially any suitable technique that is known in the art, such as depth mapping based on stereoscopic imaging or time-of-flight measurements.

In the example shown in FIG. 1, a projector 30 in imaging assembly 24 projects a pattern of optical radiation onto the scene, and a depth camera 32 captures an image of the pattern that appears on the scene (including at least the head of subject 36). A processing device in assembly 24 processes the image of the pattern in order to generate a depth map of at least a part of the body of subject 36, i.e., an array of 3D coordinates, comprising a depth (Z) coordinate value of the objects in the scene at each point (X,Y) within a predefined area. (In the context of an array of image-related data, these (X,Y) points are also referred to as pixels.) Optionally, a color camera 34 in imaging assembly 24 also captures color (2D) images of the scene, but such 2D images are not required by the methods of face detection that are described hereinbelow. Rather, the disclosed methods rely exclusively on depth information in classifying an object in the scene as a face and identifying its location.

Imaging assembly 24 generates a data stream that includes depth maps for output to an image processor, such as a computer 26. Although computer 26 is shown in FIG. 1 as a separate unit from imaging assembly 24, the functions of these two components may alternatively be combined in a single physical unit, and the depth mapping and image processing functions of system 20 may even be carried out by a single processor. Computer 26 processes the data generated by assembly 24 in order to detect the face of subject 36 and/or other subjects who may appear in the depth map. Typically, computer 26 comprises a general-purpose computer processor, which is programmed in software to carry out the above functions. The software may be downloaded to the processor in electronic form, over a network, for example, or it may alternatively be provided on tangible, non-transitory media, such as optical, magnetic, or electronic memory media. Further alternatively or additionally, at least some of the functions of computer 26 may be carried out by hard-wired or programmable logic components.

FIG. 2 is a schematic representation of a depth map captured by assembly 24, in accordance with an embodiment of the present invention. The depth map, as explained above, comprises a matrix of pixels having respective depth values. The depth values are represented in FIG. 2 as gray-scale values, with darker shades of gray corresponding to larger depth values, i.e., locations farther from assembly 24. (Black areas correspond to pixels for which no depth values could be determined.) In this particular scene, the subject has placed his hand on his head, thus obscuring some of the contours of the head.

FIG. 3 is a schematic representation of a normal map extracted from the depth map of FIG. 2 at low resolution, in accordance with an embodiment of the present invention. This normal map is computed at a low resolution level, for example 40×30 pixels, which in this case is 1/16 the size of the depth map acquired by assembly 24. Although this and the ensuing steps of the present method can also be performed at a finer resolution, it is advantageous in terms of computing speed that the initial steps (up to finding blobs in the depth map, as explained below) be performed at a coarse level of resolution.

The normal map is computed as follows: Taking u-v to be the surface parameterization grid of the depth map, p=p(u,v) represents the surface points of the depth map of FIG. 2 in 3D. Based on the depth values in this map, computer 26 calculates the cross-product of the depth gradients at each point. The result of this computation is the normal map shown in FIG. 3, in which N=N(u,v) is the surface normal at point p, so that each pixel holds a vector value corresponding to the direction of the normal to the surface defined by the depth map at the corresponding point is space. The normal vectors are difficult to show in gray-scale representation, and the normal map in FIG. 3 is therefore presented only for the sake of general illustration. Pixels whose normals are close to the Z-direction (pointing out of the page) have lighter shades of gray in FIG. 3, while those angled toward the X-Y plane are darker. In this respect, the high curvature of the head and hand can be observed in terms of the marked gray-scale gradation in FIG. 3, and this feature will be used in the subsequent steps of the analysis.

Computer 26 next computes a (low-resolution) curvature map, based on this normal map. The curvature computed for each pixel at this step can be represented in a 2×2 matrix form known in 3D geometry as the shape operator, S, which is defined as follows:

${x\; 1} = \frac{\partial p}{\partial u}$ ${x\; 2} = \frac{\partial p}{\partial v}$ $G = \begin{pmatrix} {x\; {1 \cdot x}\; 1} & {x\; {1 \cdot x}\; 2} \\ {x\; {1 \cdot x}\; 2} & {x\; {2 \cdot x}\; 2} \end{pmatrix}$ $B = \begin{pmatrix} {{\frac{\partial N}{\partial u} \cdot x}\; 1} & {{\frac{\partial N}{\partial u} \cdot x}\; 2} \\ {{\frac{\partial N}{\partial v} \cdot x}\; 1} & {{\frac{\partial N}{\partial v} \cdot x}\; 2} \end{pmatrix}$ S = B ⋅ G⁻¹

Computer 26 extracts the shape operator eigenvectors, corresponding to the two main curvature orientations, and the shape operator eigenvalues, corresponding to the curvature values along these orientations. The curvature map comprises the dominant curvature per pixel, i.e., the eigenvalue with the larger absolute value and the corresponding curvature orientation. The raw curvature value can be either positive or negative, with positive curvature corresponding to convex surface patches, and negative curvature corresponding to concave surface patches.

FIG. 4 is a schematic representation of a curvature map extracted from the normal map of FIG. 3 (and hence from depth map of FIG. 2), in accordance with an embodiment of the present invention. Due to the limitations of gray-scale graphics, this curvature map shows only the magnitude of the curvature (i.e., the dominant eigenvalue of the curvature matrix, as explained above), whereas curvature directions are shown in FIG. 6, as described below. Pixels with strongly positive curvature values have light shades of gray in the curvature map, while pixels with negative curvature values are dark gray.

Computer 26 uses the curvature map in extracting blobs having positive curvature from the original depth map. Since body parts, such as the head and hand, are inherently convex, positive curvature within a blob of pixels is a necessary condition for the blob to correspond to such a body part. Furthermore, transitions from positive to negative curvature are good indicators of the edges of a body part, even when the body part is in contact with another object without a sharp depth gradation between the body part and the object.

FIG. 5 is a schematic representation of a map of blobs extracted from the curvature map of FIG. 4, in accordance with an embodiment of the invention. The blobs due to the head and hand (which run together in FIG. 5) have strongly-positive curvature and thus can be clearly segmented from other objects based on the changes in sign of the curvature at their edges.

FIG. 6 is a schematic representation of a map of curvature direction within the blobs found in FIG. 5, in accordance with an embodiment of the invention. Computer uses the pixel-wise curvature orientations in the curvature map to find the axes of curvature of the blobs in the curvature map. The curvature vector direction, as explained above, is the direction of the major (dominant) eigenvector of the curvature matrix found in the curvature computation process. The axis of each blob is a line in the depth map (or curvature map) that runs through the center of mass of the blob in a direction perpendicular to the dominant curvature direction over the blob. This axis will be used subsequently in normalizing the classifier that is applied for face identification so as to compensate for the effect of roll, i.e., tilting the head from side to side.

Typically, computer 26 identifies the dominant curvature direction of a given blob as the statistical mode of the curvature directions of all the pixels. In other words, for each blob, the computer constructs a histogram of the curvature directions of the pixels in the blob, and identifies the dominant curvature direction as the mode of the histogram. If the histogram contains multi-modal behavior, each mode is analyzed independently, dividing the blob into multiple sub-blobs. On this basis, in the example shown in FIG. 6, the head blob, with a vertical curvature axis, is segmented from the smaller hand blob, with a diagonal curvature axis. Alternatively, other statistical averages, such as the mean or median, may be identified as the dominant curvature direction.

Having identified the blob or blobs in the depth map that are candidates to be faces, computer 26 now proceeds to process the data from these blobs in the depth map in order to decide which, if any, can be confidently classified as faces. Assuming the first phase of depth map analysis, up to identification of the candidate blobs and their axes, was performed at low resolution, as explained above, computer 26 typically processes the data in the blobs during the second, classification phase at a finer resolution. Thus, for example, FIG. 7 is a schematic representation of a normal map extracted from the depth map of FIG. 2 at a resolution of 160×120, while FIG. 8 is a schematic representation of a curvature map extracted from the normal map of FIG. 7, in accordance with an embodiment of the present invention.

Computer 26 next applies a face classifier to this curvature map. In the present embodiment, computer 26 convolves the curvature values of each blob that is to be classified with one or more filter kernels, which return a score for each pixel indicating the likelihood that it is the center point of a face. As part of this classification step, the roll angle of the face is normalized (to the vertical direction, for example) by rotating the axis derived from the curvature orientations of the pixels in the blob being classified. Additionally or alternatively, computer 26 normalizes the scale of the face based on the size of the blob. Equivalently, the filter kernel or kernels that are used in the classification may be rotated and/or scaled.

FIGS. 9A and 9B are schematic graphical representations of filter kernels used in face detection, in accordance with an embodiment of the invention. FIG. 9A represents the kernel of a face filter, which matches typical curvature features of a typical face, while FIG. 9B represents the kernel of a nose filter, which matches the high curvature values expected along the ridge of the nose. When convolved with the curvature map, these filter kernels yield a score for each pixel within the blob, indicating the log likelihood that this pixel is the center point of a face.

In addition to the nose region, additional face regions can be taken to generate a set of parts filters. This approach can be used in conjunction with a Deformable Parts Model (DPM), which performs object detection by combining match scores at both whole-object scale and object parts scale. The parts filters compensate for the deformation in the object part arrangement due to perspective changes.

Alternatively or additionally, other kernels may be used. For example, the kernels shown in FIGS. 9A and 9B are optimized for faces whose frontal plane is normal to the axis of depth camera 32, with both yaw (rotation of the head around the vertical axis) and pitch (nodding the head up and down) angles at zero. These curvature-based kernels actually have the advantage of being relatively insensitive to yaw and pitch, due to the geometrical characteristics of the face itself. In order to increase the detection range, however, additional kernels may be defined and convolved with the curvature map, corresponding to different ranges of yaw and/or pitch. For example, computer 26 may apply nine different kernels (or possibly nine pairs of face and nose kernels) corresponding to combinations of yaw=0, ±30° and pitch=0, ±30°.

FIGS. 10A and 10B are schematic graphical representations of log likelihood maps obtained by convolving the curvature map of FIG. 8 with the filter kernels of FIGS. 9A and 9B, respectively, in accordance with an embodiment of the invention. The gray scale values in these figures are proportional to the inverse of the log likelihood at each point, meaning that the darkest points in the figures corresponding to the highest log likelihood values. Computer 26 processes these maps in order to identify the blob or blobs that actually correspond to faces in the depth map. In choosing the best candidate face center points the computer considers a number of factors, for example:

-   -   Low root mean square error (RMSE) in the face kernel response at         the candidate point.     -   Highly localized face kernel response at the candidate point.     -   High curvature value at the nose location within the face (as         indicated by the nose kernel response).

In the example shown in FIGS. 10A and 10B, the filter kernels both return the same sharp peak in log likelihood at the center of the face in the depth map.

In an alternative embodiment, the principles outlined above are implemented in a deep convolutional neural network (DCNN), rather than or in addition to using explicit filter kernels as in FIGS. 9A and 9B. In this case, the input stream to the DCNN comprises the normal map and the coarse and fine level curvature maps, as described above. The roll and scale can be pre-calculated as described above and used to normalize the input streams to the DCNN. Alternatively, the input can be fed as is, letting the DCNN learn these transformations on its own. As part of the training process, the network learns the filter kernels as opposed to using fixed, “hand-crafted” kernels.

Optionally, the blobs found on the basis of curvature (as in FIG. 6) can be used as region proposals to a region-based neural network. Alternatively, the computer may further filter the depth map with the sorts of predefined filters that are described above, and then pass an even smaller set of final candidate locations to the neural network for evaluation.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. 

1. A method for processing data, comprising: receiving a depth map of a scene containing at least a humanoid head, the depth map comprising a matrix of pixels having respective pixel depth values; using a digital processor, extracting from the depth map a curvature map of the scene, the curvature map comprising respective curvature values of at least some of the pixels in the matrix; and processing the curvature values in order to identify a face in the scene.
 2. The method according to claim 1, wherein processing the curvature values comprises detecting one or more blobs in the curvature map over which the pixels have respective curvature values that are indicative of a convex surface, and identifying one of the blobs as the face.
 3. The method according to claim 2, wherein the curvature map comprises respective curvature orientations of the at least some of the pixels, and wherein identifying the one of the blobs comprises calculating a roll angle of the face responsively to the curvature orientations of the pixels in the one of the blobs.
 4. The method according to claim 3, wherein processing the curvature values comprises applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated roll angle.
 5. The method according to claim 2, wherein processing the curvature values comprises calculating a scale of the face responsively to a size of the one of the blobs, and applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated scale.
 6. The method according to claim 2, wherein extracting the curvature map comprises deriving a first curvature map from the depth map at a first resolution, and wherein detecting the one or more blobs comprises finding the one or more blobs in the first curvature map, and wherein processing the curvature values comprises deriving a second curvature map containing the one of the blobs at a second resolution, finer than the first resolution, and identifying the face using the second curvature map.
 7. The method according to claim 1, wherein processing the curvature values comprises convolving the curvature map with a curvature filter kernel in order to find a location of the face in the scene.
 8. The method according to claim 7, wherein convolving the curvature map comprises separately applying a face filter kernel and a nose filter kernel in order to compute respective candidate locations of the face, and finding the location based on the candidate locations.
 9. The method according to claim 7, wherein convolving the curvature map comprises computing a log likelihood value for each of a plurality of points in the scene, and choosing the location responsively to the log likelihood value.
 10. Apparatus for processing data, comprising: an imaging assembly, which is configured to capture a depth map of a scene containing at least a humanoid head, the depth map comprising a matrix of pixels having respective pixel depth values; and a processor, which is configured to extract from the depth map a curvature map of the scene, the curvature map comprising respective curvature values of at least some of the pixels in the matrix, and to process the curvature values in order to identify a face in the scene.
 11. The apparatus according to claim 10, wherein processing the curvature values comprises detecting one or more blobs in the curvature map over which the pixels have respective curvature values that are indicative of a convex surface, and identifying one of the blobs as the face.
 12. The apparatus according to claim 11, wherein the curvature map comprises respective curvature orientations of the at least some of the pixels, and wherein identifying the one of the blobs comprises calculating a roll angle of the face responsively to the curvature orientations of the pixels in the one of the blobs.
 13. The apparatus according to claim 12, wherein processing the curvature values comprises applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated roll angle.
 14. The apparatus according to claim 11, wherein processing the curvature values comprises calculating a scale of the face responsively to a size of the one of the blobs, and applying a curvature filter to the curvature map in order to ascertain whether the one of the blobs is the face while correcting for the calculated scale.
 15. The apparatus according to claim 11, wherein extracting the curvature map comprises deriving a first curvature map from the depth map at a first resolution, and wherein detecting the one or more blobs comprises finding the one or more blobs in the first curvature map, and wherein processing the curvature values comprises deriving a second curvature map containing the one of the blobs at a second resolution, finer than the first resolution, and identifying the face using the second curvature map.
 16. The apparatus according to claim 10, wherein processing the curvature values comprises convolving the curvature map with a curvature filter kernel in order to find a location of the face in the scene.
 17. The apparatus according to claim 16, wherein convolving the curvature map comprises separately applying a face filter kernel and a nose filter kernel in order to compute respective candidate locations of the face, and finding the location based on the candidate locations.
 18. A computer software product, comprising a non-transitory computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a depth map of a scene containing at least a humanoid head, the depth map comprising a matrix of pixels having respective pixel depth values, to extract from the depth map a curvature map of the scene, the curvature map comprising respective curvature values of at least some of the pixels in the matrix, and to process the curvature values in order to identify a face in the scene.
 19. The product according to claim 18, wherein processing the curvature values comprises detecting one or more blobs in the curvature map over which the pixels have respective curvature values that are indicative of a convex surface, and identifying one of the blobs as the face.
 20. The product according to claim 18, wherein processing the curvature values comprises convolving the curvature map with a curvature filter kernel in order to find a location of the face in the scene. 